Chromatic dispersion compensation device and method

ABSTRACT

A chromatic dispersion compensator whereby the amount of dispersion and the group delay time can be easily adjusted. A dispersion unit, a transmitting lens and a group delay generation unit are arranged along the optical axis of incident light. The dispersion unit separates the incident light into beams of respective different wavelengths. The transmitting lens is arranged across the optical paths of the beams of different wavelengths separated by the dispersion unit, and refracts the beams at different angles according to their respective incidence positions. The group delay generation unit is arranged across the optical paths of the beams of different wavelengths refracted by the transmitting lens, causes the beams to undergo propagation delay for periods corresponding to their respective incidence positions, and converges and emits the beams of different wavelengths. Consequently, the beams of different wavelengths are imparted group delay corresponding to the refracting angles of the transmitting lens.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 10/793,647filed Mar. 5, 2004, pending, which is based upon and claims the benefitof priority from prior Japanese patent application no. 2003-059555,filed Mar. 6, 2003, the entire contents of the foregoing beingincorporated herein by reference.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to a device and method for compensatingfor chromatic dispersion, and more particularly, to chromatic dispersioncompensation device and method whereby the amount of dispersion can beset as desired.

(2) Description of the Related Art

In optical communication systems, signal degradation (chromaticdispersion) occurs as light pulses are propagated over a long distancethrough an optical fiber. Accordingly, such chromatic dispersion needsto be corrected to restore the optical signal to its original state.

Generally, a dispersion compensation fiber is used to correct chromaticdispersion. The dispersion compensation fiber has a fixed amount ofdispersion, whereas the amount of chromatic dispersion of light pulseson a transmission path varies with time, depending on temperature changeetc. With the dispersion compensation fiber, therefore, it is difficultto strictly compensate for the dispersion. Moreover, present-daylarge-capacity optical communication systems require extremely strictdispersion compensation techniques, and it is difficult to meet suchrequirements with the use of the dispersion compensation fiber.

In the circumstances, a chromatic dispersion compensation device ofwhich the amount of dispersion can be set as desired has been devised.Such a dispersion compensation device will be hereinafter referred to asa VIPA (Virtually Imaged Phased Array) dispersion compensator.

FIG. 17 shows an example of a conventional VIPA dispersion compensator.The VIPA dispersion compensator comprises an optical circulator 911, anoptical fiber 912, a collimating lens 913, a line focusing lens 914, adispersion section 915, a line focusing lens 916, and a reflectingmirror section 917.

The dispersion section 915 includes a glass plate having a reflectingfilm with 100% reflectivity coated on an incidence side thereof exceptfor the light incidence area and having a high-reflectivity reflectingfilm coated on a light emission side thereof (see Unexamined JapanesePatent Publication No. H09-43057, for example). The dispersion section915 is slightly tilted with respect to the direction of incident lightfrom the line focusing lens 914. The reflecting mirror section 917comprises a mirror whose surface is curved in such a manner thatconcavity smoothly changes to convexity. In FIG. 17, the far side of themirror constitutes a concave mirror, and the near side of sameconstitutes a convex mirror.

Light incident on the optical fiber 912 from the optical circulator 911is propagated through the optical fiber 912 and then is turned into aparallel beam by the collimating lens 913. Subsequently, the parallelbeam is focused by the line focusing lens 914 to be incident on thedispersion section 915.

The incident light undergoes multiple reflection within the dispersionsection 915 and emerges therefrom. The emerging light behaves in thesame manner as light emitted from a diffraction grating and thereforeacts as diffracted light. This diffraction grating is not a real one buta virtual diffraction grating and is hence called a virtually imagedphased array (VIPA).

The diffracted light emitted in this manner has various orders ofdiffraction, and the following description is directed only to requiredorders (hereinafter “diffracted light”). The diffracted light is focusedon the reflecting mirror section 917 by the line focusing lens 916. Atthis time, light waves of different wavelengths are focused at differentlocations on the reflecting mirror section 917. The focused light wavesare reflected in various directions by the reflecting mirror section917, then pass through the line focusing lens 916 and again reach thedispersion section 915. At this time, the light waves of differentwavelengths arrive at different locations on the dispersion section 915.Consequently, the light waves require different periods of time to againreach the window of the light incidence side after undergoing multiplereflection within the dispersion section 915, thus producing group delaytime.

By moving the reflecting mirror section 917 in the X-axis direction, itis possible to adjust the incidence positions of the reflected light onthe dispersion section 915, namely, to adjust the group delay time (seeUnexamined Japanese Patent Publication No. 2003-15076, and “Design forVIPA variable dispersion compensator simulator” by Hirotomo Izumi,Yasuhiro Yamauchi, and Yuichi Kawabata, Journal C of The Institute ofElectronics, Information and Communication Engineers, Vol. J85-C, No.10, pp. 898-905, October 2002, for example).

In the conventional dispersion compensator, however, a single dispersionsection functions as both a dispersion element and a delay element, andaccordingly, optical adjustment is made taking account of the balance ofthe dispersion function and the delay function. This means that twoparameters are adjusted by means of a single parameter, and thus thereis no guarantee that both of the characteristics can always beoptimized. Namely, if the optical system is adjusted so as to obtain adesired dispersion function, then the delay function may possibly failto attain a desired value. Similarly, if the optical system is adjustedso as to obtain a desired delay function, the dispersion function maypossibly fail to attain a desired value. Thus, in the conventionaldispersion compensator, the dispersion function and the delay functionare performed by a common optical system, and this makes it difficult toattain an optimum dispersion compensation function.

SUMMARY OF THE INVENTION

The present invention was created in view of the above circumstances,and an object thereof is to provide chromatic dispersion compensationdevice and method whereby the amount of dispersion and the group delaytime can be adjusted with ease.

To achieve the object, there is provided a chromatic dispersioncompensation device for compensating for chromatic dispersion. Thechromatic dispersion compensation device comprises a dispersion unit forseparating incident light into light beams of respective differentwavelengths, a transmitting lens arranged across optical paths of thelight beams of the different wavelengths separated by the dispersionunit, for refracting the light beams of the different wavelengths atdifferent refracting angles according to respective incidence positionsof the light beams of the different wavelengths, and a group delaygeneration unit arranged across optical paths of the light beams of thedifferent wavelengths refracted by the transmitting lens, for causingthe light beams of the different wavelengths to undergo propagationdelay for time periods corresponding to respective incidence positions,and converging and emitting the light beams of the differentwavelengths.

Also, to achieve the above object, there is provided a chromaticdispersion compensation method for compensating for chromaticdispersion. The chromatic dispersion compensation method comprises thestep of separating incident light into light beams of respectivedifferent wavelengths, the step of refracting, by means of atransmitting lens arranged across optical paths of the separated lightbeams of the different wavelengths, the light beams of the differentwavelengths at different refracting angles according to respectiveincidence positions of the light beams of the different wavelengths, andthe step of causing the light beams of the different wavelengths toundergo propagation delay for time periods corresponding to therespective refracting angles of the transmitting lens.

The above and other objects, features and advantages of the presentinvention will become apparent from the following description when takenin conjunction with the accompanying drawings which illustrate preferredembodiments of the present invention by way of example.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing an arrangement of a VIPA variable dispersioncompensator according to a first embodiment of the present invention.

FIG. 2 is a diagram showing the manner of how incident light ispropagated up a dispersion section.

FIG. 3 is a diagram showing the propagation of light within thedispersion section.

FIG. 4 is a diagram showing the manner of how light is propagated from adispersion optical system to a group delay optical system.

FIG. 5 is a diagram showing the manner of how light is propagatedthrough a delay section.

FIG. 6 is a diagram showing the behavior of light within the dispersionsection.

FIG. 7 is a diagram illustrating the principles of a virtual diffractiongrating.

FIG. 8 is a diagram showing angle differences according to wavelengths.

FIG. 9 is a diagram illustrating reflection angles.

FIG. 10 is diagram illustrating the angular relationship of light beamsincident on and reflected from a reflecting mirror.

FIG. 11 is a diagram showing the manner of how light is refracted at theincidence side of a transmitting lens.

FIG. 12 is a diagram showing the manner of how light is refracted at theemission side of the transmitting lens.

FIGS. 13A and 13B exemplify angles of refraction according to incidentpositions of light on the transmitting lens, wherein FIG. 13A is aperspective view of the transmitting lens, and FIG. 13B is a side viewof the transmitting lens.

FIG. 14 illustrates the manner of how light is dispersed according tothe position of the transmitting lens, wherein part (A) of FIG. 14 is adiagram showing the case where the transmitting lens is arranged suchthat light is incident on the center of the transmitting lens withrespect to an X-axis direction, part (B) of FIG. 14 is a diagram showingthe case where the transmitting lens is arranged such that light isincident on the far side of the transmitting lens with respect to theX-axis direction, and part (C) of FIG. 14 is a diagram showing the casewhere the transmitting lens is arranged such that light is incident onthe near side of the transmitting lens with respect to the X-axisdirection.

FIG. 15 is a diagram showing an exemplary shape of the transmittinglens.

FIGS. 16A and 16B illustrate a VIPA variable dispersion compensatoraccording to a third embodiment of the present invention, wherein FIG.16A is a diagram showing an entire arrangement, and FIG. 16B is anenlarged view of a transmitting lens driving section.

FIG. 17 is a diagram showing an example of a conventional VIPAdispersion compensator.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will be hereinafter described withreference to the drawings.

First Embodiment

FIG. 1 shows an arrangement of a VIPA variable dispersion compensatoraccording to a first embodiment of the present invention. The VIPAvariable dispersion compensator of the first embodiment comprises adispersion unit 10 and a group delay generation unit 20 which are soarranged as to be in the relationship of mirror symmetry with respect toa transmitting lens 1. Compared with the conventional VIPA variabledispersion compensator, the transmitting lens 1 is used in place of thereflecting mirror section and two optical devices, namely, thedispersion unit 10 and the group delay generation unit 20, are arrangedon both sides of the transmitting lens 1.

The dispersion unit 10, the transmitting lens 1 and the group delaygeneration unit 20 are arranged along the optical path of light incidenton the VIPA variable dispersion compensator. In FIG. 1, X axis indicatesthe direction from the far side to near side in the figure, Y axisindicates the bottom-to-top direction in the figure, and Z axisindicates the left-to-right direction in the figure. In the illustratedexample, the incident light enters the compensator in a direction fromthe negative side to positive side of the Z axis.

Specifically, the dispersion unit 10 includes a first collimating lens11, a first line focusing lens 12, a dispersion section 13 and a firstfocusing lens 14 arranged in the order mentioned from the side of anoptical fiber 2 toward the transmitting lens 1. The first collimatinglens 11 collimates (obtains a parallel beam from) light emerging fromthe optical fiber 2. The first line focusing lens 12 focuses thecollimated light into a line segment of light within an incidence windowof the dispersion section 13. The first collimating lens 11 and thefirst line focusing lens 12 constitute a collimating lens section.

The dispersion section 13 comprises a transparent parallel flat platesuch as a glass plate, a reflecting film with 100% reflectivity coatedon a light incidence side of the flat plate except for the incidencewindow, and a reflecting film with high reflectivity (e.g., about 95 to98%) coated on an emission side of the flat plate. The parallel flatplate may be transparent only to that wavelength band of light withrespect to which the VIPA variable dispersion compensator is used. Thefirst collimating lens 11, the first line focusing lens 12 and the firstfocusing lens 14 have their incidence planes directed substantiallyperpendicularly to the optical axis (parallel to the Z axis) of theincident light, but the dispersion section 13 is tilted relative to theoptical axis.

The first focusing lens 14 focuses diffracted light from the dispersionsection 13 on a straight line (parallel to the Y axis) where thetransmitting lens 1 is arranged.

The transmitting lens 1 is an aspherical lens made of a material whichis transparent to the wavelength band used. The transmitting lens 1 hasan optical characteristic whereby light is refracted in a manner suchthat the light incident thereon from the dispersion unit 10 is emittedat an angle identical with that at which light is reflected by thereflecting mirror used in the conventional VIPA variable dispersioncompensator (the light is, however, emitted in an opposite direction,i.e., on the positive side of the Z axis).

For example, the transmitting lens 1 has an optical characteristic suchthat, when the light incidence position is shifted in the X-axisdirection with the Y coordinate value unchanged, the refracting anglechanges continuously (or stepwise). Specifically, the near side of thetransmitting lens 1, as viewed in FIG. 1, has a sectional form showingan optical characteristic similar to that of a diverging lens (concavelens) and the far side of the lens 1 has a sectional form showing anoptical characteristic similar to that of a converging lens (convexlens). By moving the transmitting lens 1 having such a shape in theX-axis direction, it is possible to shift the incidence position on theincidence plane of the transmitting lens 1, and consequently, to controlthe degree of divergence or convergence of the incident light. Namely,the transmitting lens 1 can function as the reflecting mirror of theconventional VIPA variable dispersion compensator.

The group delay generation unit 20 includes a second focusing lens 21, adelay section 22, a second line focusing lens 23 and a secondcollimating lens 24 arranged in the order mentioned from the side of thetransmitting lens 1 toward an optical fiber 3. The second focusing lens21 turns a plurality of diffracted light beams of respective differentwavelengths, emitted from the transmitting lens 1, to parallel beams oflight.

The delay section 22 comprises a parallel flat plate transparent to thewavelength band used, such as a glass plate, a reflecting film with highreflectivity (e.g., about 95 to 98%) coated on a light incidence side ofthe flat plate, and a reflecting film with 100% reflectivity coated on alight emission side of the flat plate except for an emission window. Thesecond focusing lens 21, the second line focusing lens 23 and the secondcollimating lens 24 have their incidence planes directed substantiallyperpendicularly to the optical axis (parallel to the Z axis) of theincident light, but the delay section 22 is tilted relative to theoptical axis.

The second line focusing lens 23 turns the light emerging from the delaysection 22 into a parallel beam of light. The second collimating lens 24focuses the collimated light on the core of the optical fiber 3. Thesecond collimating lens 24 and the second line focusing lens 23constitute a collimating lens section.

In the VIPA variable dispersion compensator constructed as describedabove, light is made incident on the dispersion unit 10 from the opticalfiber 2. Then, in the dispersion unit 10, the incident light isdispersed according to wavelengths and focused on the transmitting lens1 in a straight line parallel to the Y axis. At this time, the lightbeams of respective different wavelengths are focused at positions(“bright” fringe positions in the interference fringes) shifted in theY-axis direction according to their wavelengths.

Specifically, the light focused on the dispersion section 13 by thecollimating lens section emerges only from the emission side of thedispersion section without leaking from the incidence side, because theincidence side of the dispersion section 13 except the incidence windowis coated with a film having 100% reflectivity while the emission sideof same is coated with a film having a different but high reflectivity.At this time, the incident light undergoes multiple reflection insidethe dispersion section 13 and gradually emerges therefrom; accordingly,the dispersion section behaves in the same manner as a diffractiongrating. The dispersion section 13 has interference conditions imposedon light beams to be emitted in respective directions, and light beamssatisfying the interference conditions are dispersed and emitted intheir respective directions. Also, the dispersion section 13 has athickness designed such that the FSR (Free Spectral Range) thereofmatches the wavelength spacing used in a WDM (Wavelength DivisionMultiplexing) optical communication system, and accordingly, thenecessary light is allowed to pass through the dispersion section.

The light beams emitted from the dispersion section 13 impinge on thefirst focusing lens 14 at different angles according to their respectivewavelengths and are focused by the lens 14. In this case, the lightbeams are focused into a straight line on a plane parallel to adirection (Y-axis direction) perpendicular to the optical axis but atdifferent positions on the straight line according to their respectivewavelengths. The transmitting lens 1 is arranged on the straight linewhere the light beams of different wavelengths are focused, so that thefocused light beams of different wavelengths are incident on differentpositions of the transmitting lens 1.

The transmitting lens 1 refracts the input light and emits the refractedlight to the group delay generation unit 20. Specifically, the lightbeams focused into a straight line parallel to the Y axis are refractedat different refracting angles according to their respective focusingpositions, and the refracting angles are designed such that the finalgroup delay time characteristic shows a linear function.

In this case, the transmitting lens 1 exhibits different lenscharacteristics (refracting angles) with respect to the incident lightbeams, depending on where along the X axis the light beam enters thelens 1. When the light beams are incident on a position where thetransmitting lens has an optical characteristic similar to that of adiverging lens, the light beams of respective different wavelengthsenters the group delay generation unit 20 at large incidence intervals.When the light beams are incident on a position where the transmittinglens has an optical characteristic similar to that of a converging lens,the light beams of respective different wavelengths enters the groupdelay generation unit 20 at small incidence intervals. Where the degreeof convergence is greater than a certain level, the incidence positionsof the light beams of respective wavelengths on the group delaygeneration unit 20 are inverted along the Y axis. If the degree ofconvergence is even greater, the incidence intervals of the respectivewavelengths become large.

The group delay generation unit 20 subjects the incident light beams togroup delay according to their respective incidence positions andoutputs the resultant light to the optical fiber 3. Namely, the largerthe incidence intervals of the respective wavelengths, the larger groupdelay time differences are produced among the respective wavelengths.The incidence positions on the group delay generation unit 20 aredependent on the refracting angles of the transmitting lens 1, andtherefore, the light beams of different wavelengths are subjected torespective different propagation delays corresponding to the refractingangles of the transmitting lens 1.

Specifically, the light beams refracted in a direction parallel to theoptical axis by the transmitting lens 1 are propagated to the secondfocusing lens 21. The second focusing lens 21 passes the incoming lightbeams therethrough to turn same to parallel beams, which are thenemitted to the delay section 22. The delay section 22 satisfies theinterference conditions for the light beams emitted from the dispersionsection 13, and accordingly, the light beams incident on the delaysection 22 undergo multiple reflection within the delay section 22 andemerge from the emission window in linear form. The number of times thelight beams are reflected varies depending on the wavelengths, andaccordingly, the light beams of different wavelengths require differentperiods of time to reach the emission window of the delay section 22. Itis therefore possible to produce chromatic dispersion.

In the conventional VIPA dispersion compensator, the dispersion functionand the delay line function are adjusted by means of a single dispersionsection, but with the aforementioned arrangement, these functions can beseparately adjusted by means of the dispersion section and the delaysection, thus making it possible to improve the characteristics of thecompensator. Namely, the dispersion section 13 disperses light and thedelay section 22 subjects the light to group delay for propagation, sothat the dispersion and the group delay can be adjusted independently ofeach other. For example, after the thickness and incidence angle of thedispersion section 13 are adjusted so as to obtain an optimum dispersionfunction, the delay time (plate thickness or incidence angle) of thedelay section 22 can be adjusted without taking account of lowering ofthe dispersion function. Since the dispersion function and the delayfunction can be adjusted independently of each other, thecharacteristics of the compensator can be adjusted with ease.

The behavior of light input to the VIPA variable dispersion compensatorwill be now described in more detail.

FIG. 2 shows the manner of how incident light is propagated to thedispersion section. The dispersion section 13 has a parallel flat plate13 a and a reflecting film 13 b affixed to an upper part of theincidence side (left side in FIG. 2) of the flat plate. The reflectingfilm 13 b has a reflectivity of nearly 100%. An incidence window islocated on the incidence side below the reflecting film 13 b and isaffixed with an antireflection film 13 c having a reflectivity of nearly0% and thus capable of transmitting light therethrough. Theantireflection film 13 c may be an AR (Anti Reflection) coat, forexample. The boundary between the reflecting film 13 b and theantireflection film 13 c is in the form of a straight line. Also, theemission side (right side in FIG. 2) of the parallel flat plate 13 a isaffixed with a reflecting film having high reflectivity (e.g., 95 to98%) and capable of transmitting only a small amount of lighttherethrough.

Incident light 31 emitted from the optical fiber 2 enters the firstcollimating lens 11. The first collimating lens 11 turns the incidentlight 31 into parallel light, which is then emitted to the first linefocusing lens 12. The parallel incident light 31 is focused by the firstline focusing lens 12 into a straight line in the vicinity of theboundary between the reflecting film 13 b on the antireflection film 13c formed on the surface of the dispersion section 13 and theantireflection film 13 c.

FIG. 3 shows the propagation of light within the dispersion section 13.The dispersion section 13 is constituted by the parallel flat plate 13 a(with a thickness of t), the reflecting film 13 b, the antireflectionfilm 13 c and a reflecting film 13 d, as mentioned above.

The incident light 31 is focused (on the antireflection film 13 c) inthe vicinity of the boundary between the reflecting film 13 b and theantireflection film 13 c on the incidence side, whereupon the incidentlight 31 transmits through the parallel flat plate 13 a while dispersingtherein. Some light beams 32 a, 32 b pass through the reflecting film 13d to outside while the other light beams are reflected by the reflectingfilm 13 d. The reflected light beams are then reflected by thereflecting film 13 b and again reach the reflecting film 13 d, whereuponsome light beams 33 a, 33 b pass through the reflecting film 13 d tooutside while the other light beams are reflected by the reflecting film13 d. The reflected light beams travel back and forth within theparallel flat plate 13 a and some light beams 34 a, 34 b pass throughthe reflecting film 13 d to outside.

In FIG. 3, two directions (first direction, second direction) areillustrated as directions in which beams of light (light beams) areemitted on reaching the reflecting film 13 d. In practice, light beamsare emitted in various directions within a predetermined angular rangeincluding the illustrated directions.

As shown in FIG. 3, the incident light 31 is emitted while beingsubjected to multiple reflection within the parallel flat plate 13 a, sothat the dispersion section 13 can be imparted a dispersion functionsimilar to that of a diffraction grating. Namely, interference fringes(spectrum) of the respective wavelengths are produced at infinity.

The emergence angles of interference light beams of respectivewavelengths are determined by the optical path differences resultingfrom the back-and-forth traveling of the light beams inside the parallelflat plate 13 a. For example, in the case of the light beams 32 a and 33a (emitted in the first direction), the optical path of the light beam33 a is longer by an amount corresponding to one back-and-forth travelwithin the parallel flat plate 13 a. Provided this optical pathdifference is an integer multiple of the wavelength of a light beamcontained in the incident light 31, when the light beams 32 a and 33 aare focused by a convex lens (focusing lens 14), the focused positionconstitutes a bright fringe in the interference fringes. At the brightfringe, only light beams with wavelengths satisfying the interferencecondition are focused.

The light beams 32 b and 33 b (emitted in the second direction) alsoproduce interference light. The light beam 32 b is incident on thereflecting film 13 d at a larger incidence angle (angle formed betweenthe incident light beam and the normal line normal to the reflectingsurface 13 d) than the light beam 32 a. Accordingly, the optical pathdifference between the light beams 32 b and 33 b (second direction) issmaller than that between the light beams 32 a and 33 a (firstdirection), so that the light beams emitted in the second directionproduce interference light of wavelengths different from those of thelight beams emitted in the first direction. Thus, the optical pathdifference varies depending on the direction of emission of light, andalso the wavelengths of light beams producing interference light (brightfringe) vary depending on the emission direction, causing chromaticdispersion as a result. The dispersed light is propagated to the groupdelay generation unit 20 through the transmitting lens 1.

FIG. 4 shows the manner of how light is propagated from the dispersionoptical system to the group delay optical system. In FIG. 4, the lightemitted from the dispersion section 13 is indicated by light beams 41 to43 according to their respective emission directions. The light beams 41to 43 are focused by the first focusing lens 14 on the transmitting lens1.

The light beam 41 is propagated in an obliquely upward direction(positive direction of both Y and Z axes) to be incident on the firstfocusing lens 14, then converged by the first focusing lens 14, andfocused on an upper part of the transmitting lens 1 as a light beam 41a. The light beam 42 is propagated in a horizontal direction (parallelto the Z axis) to be incident on the first focusing lens 14, thenconverged by the first focusing lens 14, and focused on the center ofthe transmitting lens 1 as a light beam 42 a. The light beam 43 ispropagated in an obliquely downward direction (negative direction of theY axis and positive direction of the Z axis) to be incident on the firstfocusing lens 14, then converged by the first focusing lens 14, andfocused on a lower part of the transmitting lens 1 as a light beam 43 a.

A plurality of wavelengths contained in each of the light beams 41 to 43interfere with one another while being propagated. Thus, in each of thelight beams 41 to 43, only the components of light with wavelengthssatisfying the interference condition intensify their brightness, whilethe components of light with other wavelengths darken. As a result, wheneach of the light beams 41 a, 42 a and 43 a is focused on thetransmitting lens 1, only the components of light with the wavelengthssatisfying the interference condition for a bright fringe constitute abright fringe at the focused position. The light beams 41 a to 43 a arepropagated thereafter as light beams with the wavelengths satisfying therespective interference conditions.

The light beams 41 a, 42 a and 43 a focused on the transmitting lens 1are diverged or converged by the lens 1 to be incident on the secondfocusing lens 21 as light beams 41 b, 42 b and 43 b, respectively. Thelarger the degree of divergence (or the smaller the degree ofconvergence) of the transmitting lens 1, the higher the incidenceposition of the light beam 41 b shifts and the lower the incidenceposition of the light beam 43 b shifts. Also, the smaller the degree ofdivergence (or the larger the degree of convergence) of the transmittinglens 1, the lower the incidence position of the light beam 41 b shiftsand the higher the incidence position of the light beam 43 b shifts.

The light beams 41 b, 42 b and 43 b incident on the second focusing lens21 are turned into parallel light beams 41 c, 42 c and 43 c,respectively, by the lens 21 to be incident on the delay section 22.

FIG. 5 shows the manner of how light is propagated through the delaysection. The delay section 22 has a construction identical with that ofthe dispersion section 13 but the incidence and emission sides arereversed. Specifically, a reflecting film 22 b with 100% reflectivity isaffixed to an upper part of the emission side of the parallel flat plate22 a, and an antireflection film 22 c capable of transmitting lighttherethrough is affixed to an emission window located below thereflecting film 22 b. The incidence side is affixed with a reflectingfilm 22 d having high reflectivity (e.g., 95 to 98%) and thus capable oftransmitting only a small amount of light.

The delay section 22 satisfies the interference conditions for thewavelengths of light beams 51, 52 incident thereon. If the dispersionsection 13 and the delay section 22 are tilted at the same angle (but inopposite directions), the thickness D of the delay section 22 should bet, or 2×N×t, where N is an integer greater than or equal to “1” and t isthe thickness of the dispersion section 13. Where the dispersion section13 has an FSR of 100 GHz, for example, the delay section 22 may have anFSR for 50 GHz or 25 GHz, so that the interference conditions for outputlight can be satisfied.

The delay section 22 is tilted with respect to the direction of incominglight so that light incident on the upper part of the delay section,which is affixed with the reflecting film 22 b, may be propagateddownward toward the antireflection film 22 c serving as the emissionwindow.

The light beams 51, 52 with different wavelengths are incident on thedelay section 22. In the example shown in FIG. 5, the light beam 51enters the delay section at an higher position than the light beam 52.The incident light beams 51, 52 are propagated within the parallel flatplate 22 a while being repeatedly reflected between the reflecting films22 b and 22 d. On arrival at the antireflection film 22 c, the lightbeams 51, 52 are emitted through the film 22 c. The emitted light 53contains components with a plurality of wavelengths.

Since the incidence positions of the light beams 51 and 52 are differentfrom each other, the light beams need to be propagated over differentdistances until they reach the emission position. Because of thedifference in the distance, propagation delay occurs, thus producinggroup delay times for respective wavelengths.

The emitted light 53 is incident on the second line focusing lens 23 andturned into parallel light. The parallel light 53 is then focused by thesecond collimating lens 24 to be incident on the optical fiber 3.

The dispersion unit 10 and the group delay generation unit 20 areseparately provided as mentioned above, and accordingly, the chromaticdispersion characteristic can be adjusted separately from the groupdelay time characteristic. As a result, the VIPA variable dispersioncompensator can be easily adjusted so that the chromatic dispersion andthe group delay time may be greatest.

In the dispersion unit 10, for example, the light dispersioncharacteristic may be precisely adjusted so that the propagated lightmay clearly show interference fringes of desired orders. Also, since thedispersion characteristic can be set without regard to the group delaytime, the dispersive power (ratio of change in dispersion angle(diffraction angle) to change in wavelength λ) can be easily made to beextremely large, or conversely, be small.

The group delay generation unit 20 may be adjusted taking account ofonly the group delay time of input light, and thus can be adjusted tohave a characteristic such that an extremely long group delay time isgenerated. For example, since the delay section 22 may have a thicknessdifferent from that of the dispersion section 13, the thickness of thedelay section 22 may be increased to thereby increase the group delaytime. The delay section 22 needs to fulfill the interference conditionsfor output light. Accordingly, the thickness of the delay section 22should be equal to the thickness t of the dispersion section 13 or be 2ntimes as large as t.

Also, the amount of the group delay time of the VIPA variable dispersioncompensator can be adjusted by shifting the position along the X axis ofthe transparent transmitting lens 1 arranged at the center. How long thegroup delay time needs to be set depends on the transmission path overwhich light has been propagated before being input to the VIPA variabledispersion compensator. Accordingly, when the VIPA variable dispersioncompensator is connected to the transmission path, the technicianadjusts the position of the transmitting lens 1 along the X axis so thata desired group delay time may be generated.

A design method for the transmitting lens 1 capable of controlling thegroup delay time will be now described. The refractive indices atrespective incidence positions of the transmitting lens 1 are determinedin accordance with the group delay times to be generated for respectivewavelengths (so that the group delay time characteristic may show alinear function, for example). Thus, the optical characteristic of thetransmitting lens 1 is not so simple as in the case of an opticalelement having a single focal point. The following describes in detailthe design method for the transmitting lens 1.

First, the behavior of light in the dispersion section 13 will beconsidered.

FIG. 6 shows the behavior of light within the dispersion section. In thefollowing, Θ represents the incidence angle of light incident on thedispersion section 13 and n represents the refractive index of theparallel flat plate 13 a of the dispersion section 13. Provided theemergence angle is θ, the relationship of

sin Θ=n sin Θ  (1)

stands in accordance with Snell's law. The incidence angle is small andthus linearly approximated, then

Θ≈nθ  (2)

Light undergoes repeated multiple reflection within the dispersionsection 13 while enlarging its beam diameter. This behavior of light isequivalent to that of light emitted from a transmission-type stepdiffraction grating; therefore, the section 13 acts as a dispersionunit. Consequently, light beams are emitted from the dispersion section13 in a manner dispersed according to respective different wavelengths.

The behavior of light at the time of emission will be now considered.Provided Φ is an angle at which light is emitted from the dispersionsection 13 and φ is an incidence angle of light to the reflecting film13 d when the light emerges from the dispersion section 13, then

n sin φ=sin φ  (3)

stands according to Snell's law. This equation is approximated, like(2), then

nφ≈Φ  (4)

The following focuses on two points concerning the virtually imagedphased array (virtual diffraction grating).

FIG. 7 illustrates the principles of such a virtual diffraction grating.In FIG. 7, comparison is made between a light beam 54 which is outputwithout being reflected even once and a light beam 55 which is outputafter making one back-and-forth travel due to reflection within thedispersion section 13. When observed from the emission side, the lightbeam 54 is equivalent to a light beam emitted from a light source 61located at the incidence position. The light beam 55 is propagated foran extra distance corresponding to one back-and-forth travel within thedispersion section 13. Accordingly, when observed from the emissionside, the light beam 55 is equivalent to a light beam emitted from alight source 62 which is located farther than the light source 61 by anamount equal to the thickness of two parallel flat plates. Thedifference in distance between the propagation paths of the light beams54 and 55 is 2t cos φ.

Provided the wavelength of the light beams 54 and 55 is λ and the orderof generated interference fringe is m, the condition for intensifyingthe two light beams 54 and 55 at infinity is given by

2nt cos φ=mλ  (5)

in consideration of the refractive index n of the parallel flat plate 13a. The left side of the equation (5) indicates the optical pathdifference and the right side of same indicates the interferencecondition (condition for bright fringe). With respect to the same order,a plurality of light beams with different wavelengths are compared, thenφ decreases with increase in wavelength.

FIG. 8 shows angle differences according to wavelengths. As seen fromFIG. 8, a light beam 56 a with a shorter wavelength than a light beam 56shows an increased φ while a light beam 56 b with a longer wavelengthshows a decreased φ.

Angle dispersion, which is a measure of the dispersion performance ofthe dispersion section 13, can be obtained by using equation (5).Namely, if the angle φ is within the range of linear approximation, therelationship

$\begin{matrix}\begin{matrix}{{d\; \varphi} = {{- \cos}\mspace{11mu} {\varphi \cdot \frac{d\; \lambda}{\lambda}}}} \\{\approx {{- \frac{1}{\varphi}} \cdot \frac{d\; \lambda}{\lambda}}}\end{matrix} & (6)\end{matrix}$

holds. Further, using the expression (4) provides

$\begin{matrix}{\frac{\Phi}{\lambda} \approx {- \frac{n^{2}}{\lambda\Phi}}} & (7)\end{matrix}$

Now, the traveling direction of light reflected by the reflecting mirrorused in the conventional VIPA variable dispersion compensator will beconsidered.

Provided the curvature of the reflecting mirror is C(y), the inclinationh(y) of the mirror is given by

$\begin{matrix}{{h(y)} = \frac{{c(y)}}{y}} & (8)\end{matrix}$

The function C(y) indicative of the curvature of the reflecting mirroris determined such that the group delay time characteristic (group delaytime difference relative to wavelength difference) shows a linearfunction.

The reflection angle of light will be now considered by tracing a lightbeam with a single wavelength emitted from the dispersion section 13 andreflected by the reflecting mirror.

FIG. 9 illustrates reflection angles. It is assumed that a light beam 57is emitted from a point where a line 57 b passing through the center ofthe focusing lens 14 and parallel with the Z axis intersects with thedispersion section 13. The light beam 57 passes through the focusinglens 14 and reaches a reflecting mirror 901. A dashed line passingthrough the center of the focusing lens 14 is an auxiliary line 57 aparallel with the light beam 57 falling on the focusing lens 14.

A light beam is not refracted when passed through the center of thefocusing lens 14. However, the light beam 57 passes through a region ofthe focusing lens 14 other than the center; therefore, the beam 57 isrefracted and focused on the reflecting mirror 901. Thus, parallel beamsof light are focused on the same point.

Provided the distance from the dispersion section 13 to the focusinglens 14 is a and the distance from the focusing lens 14 to thereflecting mirror 901 is f, a difference H1 in the Y-axis directionbetween the position of incidence of the light beam 57 on the focusinglens 14 and the center of the focusing lens 14 is given by

H1=a·tan(Φ−Θ)≈a(Φ−Θ)  (9)

A difference H2 in the Y-axis direction between the emission position ofthe light beam 57 from the focusing lens 14 and the incidence positionof the beam 57 on the reflecting mirror 901 is

$\begin{matrix}\begin{matrix}{{H\; 2} = {{f\; {\tan \left( {\Phi - \Theta} \right)}} - {{atan}\left( {\Phi - \Theta} \right)}}} \\{\approx {{f\left( {\Phi - \Theta} \right)} - {a\left( {\Phi - \Theta} \right)}}} \\{= {\left( {f - a} \right)\left( {\Phi - \Theta} \right)}}\end{matrix} & (10)\end{matrix}$

The behavior of light reflected by the reflecting mirror 901 will be nowconsidered.

FIG. 10 illustrates the angular relationship of light beams incident onand reflected from the reflecting mirror. It is assumed that the lightbeam 57 emitted from the focusing lens 14 is inclined at an angle β withrespect to the Z axis, and that a surface region of the reflectingmirror 901 where the light beam 57 falls on the mirror 901 is inclinedat an angle γ with respect to the Y axis. The angle γ can be expressedas follows:

γ≈tan γ≈h(y)  (11)

The angle β can be expressed as follows:

$\begin{matrix}\begin{matrix}{\beta \approx {\tan \mspace{11mu} \beta}} \\{= \frac{H\; 2}{f}} \\{= \frac{\left( {f - a} \right) \cdot \left( {\Phi - \Theta} \right)}{f}}\end{matrix} & (12)\end{matrix}$

From the above, the reflection angle of the light beam 57 reflected bythe reflecting mirror 901 can be obtained. The present invention usesthe transmitting lens 1 in place of the reflecting mirror 901, andtherefore, the shape of the transmitting lens 1 may be designed suchthat the light beam 57 is emitted at the same angle as the angle ofreflection of the beam by the reflecting mirror 901 (but in oppositedirections along the Z axis).

It is assumed that the transmitting lens 1 has a refractive index n1.Then, an incidence surface shape Z(y) is obtained by which the lightbeam 57 incident on the transmitting lens 1 is refracted at such arefracting angle as to travel thereafter in a direction parallel to theoptical axis (Z axis).

FIG. 11 shows the manner of how light is refracted at the incidence sideof the transmitting lens. In FIG. 11, the traveling direction of thelight beam 57 is identical with that shown in FIG. 10. Provided therefracting angle (angle between the refracted light beam and a normalline passing through the incidence position) is ζ, the relationship of

sin(β+ζ)=n1 sin ζ  (13)

stands in accordance with Snell's law. The angle is very small and thuslinearly approximated, then

β+ζ=n1ζ  (14)

This equation is modified to be a variable of y, by using y=f(Φ−Θ) andthe equation (14), then

$\begin{matrix}{\frac{{Z(y)}}{y} = {\frac{1}{{n\; 1} - 1}\frac{\left( {f - a} \right)}{f^{2}y}}} & (15)\end{matrix}$

With the differential of Z(y) approximated to ζ, the equation (15) issolved for Z(y), then

$\begin{matrix}{{Z(y)} = {\frac{1}{{n\; 1} - 1}\frac{\left( {f - a} \right)}{f^{2}}\log \mspace{11mu} y}} & (16)\end{matrix}$

In this manner, the incidence surface shape can be obtained whereby thelight beam 57 incident on the transmitting lens 1 can be refracted in adirection parallel to the optical axis.

Where the angle is measured from the optical axis in such a way that theangle increases in a clockwise direction, it is necessary that, fromFIG. 11, light should be emitted from the emission surface of thetransmitting lens 1 in the direction of

$\begin{matrix}{{\beta + {2\gamma}} = {\frac{\left( {f - a} \right)\left( {\Phi - \Theta} \right)}{f} + {2{h(y)}}}} & (17)\end{matrix}$

Here, a model is considered wherein light is incident on the emissionsurface in a direction parallel to the Z axis.

FIG. 12 shows the manner of how light is refracted at the emission sideof the transmitting lens. The light beam 57 transmitted through thetransmitting lens 1 is refracted at the emission surface. At this time,the light beam needs to be emitted in the direction indicated by theequation (17). In order for the light beam to be emitted in such adirection, it is necessary that the relationship of

$\begin{matrix}{{n\; 1\sin \; \delta} = \left( {\frac{\left( {f - a} \right)\left( {\Phi - \Theta} \right)}{f} + {2{h(y)}}} \right)} & (18)\end{matrix}$

should be fulfilled, where n1 is the refractive index of the transparentmaterial at the emission surface and δ is the incidence angle.

In FIG. 12, δ is equivalent to the inclination of the transmitting lens1 inclined at δ with respect to the optical axis, and thus the emissionsurface shape X(y) of the transparent material can be expressed as

$\begin{matrix}{\frac{{x(y)}}{y} \approx \delta} & (19)\end{matrix}$

From the relationship y=f(Φ−Θ) and the fact that h(y) is the result ofdifferentiation of the reflecting mirror shape C(y), the equation (18)can be reduced to

$\begin{matrix}{\frac{{x(y)}}{y} = {\frac{1}{\left( {{n\; 1} - 1} \right)}\left\{ {\frac{\left( {f - a} \right)}{f^{2}y} + {2\frac{{C(y)}}{y}}} \right\}}} & (20)\end{matrix}$

This equation is solved, then

$\begin{matrix}{{X(y)} = {\frac{1}{\left( {{n\; 1} - 1} \right)}\left\{ {{\frac{\left( {f - a} \right)}{f^{2}}\log \mspace{11mu} y} + {2{C(y)}}} \right\}}} & (21)\end{matrix}$

The shape of the emission surface may be determined so as to fulfill therelationship of the equation (21).

The foregoing is the concept of design for the transparent material. Thereflecting mirror shape C(y) is determined such that the group delaytime characteristic shows a linear function (the group delay timecharacteristic is expressed as a straight line on a graph whosehorizontal axis indicates wavelength difference and whose vertical axisindicates group delay time difference). Accordingly, the emissionsurface shape is determined in accordance with the equation (21),whereby the VIPA variable dispersion compensator of this embodiment canbe imparted a group delay characteristic showing a linear function.

The aforementioned example is designed such that light is propagatedthrough the transmitting lens 1 in a direction parallel to the opticalaxis. It is, however, apparent that the transmitting lens may beconfigured in different ways, since what is essential is that the lightemitted from the transmitting lens 1 be directed as desired.

In this manner, the amount of dispersion can be controlled by the shapeof the emission side of the transmitting lens 1. Accordingly, bycontinuously varying the coefficient part of the shape function,represented by C(y), in the X-axis direction of the transmitting lens 1,it is possible to adjust the refracting angle of light emitted from thetransmitting lens 1 in accordance with the light incidence position. Forexample, the group delay time characteristic which is dependent on therefracting angle is adjusted so as to show a linear function whoseinclination (amount of change in group delay time per unit wavelengthdifference) continuously varies along the transmitting lens 1 in theX-axis direction.

FIGS. 13A and 13B exemplify angles of refraction according to incidentpositions of light on the transmitting lens, wherein FIG. 13A is aperspective view of the transmitting lens, and FIG. 13B is a side viewof the transmitting lens.

FIG. 13A shows three optical axes 58 a, 58 b and 58 c of light beamsincident on the transmitting lens 1. The incidence positions of theoptical axes 58 a, 58 b and 58 c on the transmitting lens 1 are shiftedfrom one another in the X-axis direction; however, the travelingdirections of the optical axes 58 a, 58 b and 58 c are the same and alsotheir incidence positions on the transmitting lens 1 along the Y axisare the same. Because of the difference in the incidence position of theoptical axes 58 a, 58 b and 58 c along the X axis, emitted light beamsare propagated in different directions.

In the transmitting lens 1 shown FIG. 13B, a far side of the lens asviewed in the figure (negative side of the X axis) has an opticalcharacteristic similar to that of a diverging lens, the degree ofdivergence lessens toward the near side, and the nearest side of thelens (positive side of the X axis) has an optical characteristic similarto that of a converging lens. In this case, the light beam 58 a incidenton the transmitting lens 1 at the negative side of the X axis ispropagated through the lens 1 in a horizontal direction (parallel to theZ axis) and then refracted in a positive direction of the Y axis. Thelight beam 58 b incident on the transmitting lens 1 near the centerthereof is propagated through the lens 1 in a horizontal direction(parallel to the Z axis) and then emitted without being refracted. Thelight beam 58 c incident on the transmitting lens 1 at the positive sideof the X axis is propagated through the lens 1 in a horizontal direction(parallel to the Z axis) and then refracted in a negative direction ofthe Y axis.

The transmitting lens 1 designed in this manner is moved in the X-axisdirection within the VIPA variable dispersion compensator, whereby theamount of divergence (or the amount of convergence) of light emittedfrom the lens 1 can be adjusted.

FIG. 14 illustrates the manner of how light is dispersed according tothe position of the transmitting lens, wherein part (A) of FIG. 14 is adiagram showing the case where the transmitting lens is arranged suchthat light is incident on the center of the transmitting lens withrespect to the X-axis direction, part (B) of FIG. 14 is a diagramshowing the case where the transmitting lens is arranged such that lightis incident on the far side of the transmitting lens with respect to theX-axis direction, and part (C) of FIG. 14 is a diagram showing the casewhere the transmitting lens is arranged such that light is incident onthe near side of the transmitting lens with respect to the X-axisdirection. In FIG. 14, a light beam 71 indicated by the solid line isincident on the transmitting lens 1 near the center thereof along the Yaxis, and a light beam 72 indicated by the dashed line is incident on anupper portion of the transmitting lens 1 with respect to the centerthereof along the Y axis.

As shown in part (A) of FIG. 14, the light beams 71 and 72 incident onthe transmitting lens 1 are emitted from the lens 1 at respectivepredetermined angles and then turned to parallel beams by the secondfocusing lens 21 to be incident on the delay section 22.

If the transmitting lens 1 is moved toward the near side (positivedirection of the X axis) as shown in part (B) of FIG. 14, the light beam72 is incident on a position similar to that of the optical axis 58 ashown in FIG. 13A. In this case, the incidence position of the lightbeam 72 on the delay section 22 is shifted upward, compared with theposition before the movement of the transmitting lens 1. Accordingly,the light beam 72 needs to follow a prolonged optical path until it isemitted from the delay section 22, so that the group delay timeincreases.

On the other hand, if the transmitting lens 1 is moved toward the farside (negative direction of the X axis) as shown in part (C) of FIG. 14,the light beam 72 is incident on a position similar to that of theoptical axis 58 c shown in FIG. 13A. In this case, the incidenceposition of the light beam 72 on the delay section 22 is shifteddownward, compared with the position before the movement of thetransmitting lens 1. Accordingly, the light beam 72 follows a shortenedoptical path until it is emitted from the delay section 22, so that thegroup delay time decreases.

Thus, the group delay time can be adjusted by moving the transmittinglens 1. Where the VIPA variable dispersion compensator is connected toan optical fiber, light emerging from the optical fiber shows a degreeof chromatic dispersion that varies depending on the transmission pathetc. Accordingly, the technician who installs the VIPA variabledispersion compensator shifts the position of the transmitting lens 1 inthe X-axis direction to adjust the group delay time, therebycompensating for the chromatic dispersion.

Second Embodiment

In a second embodiment, the transmitting lens has a different exemplaryshape.

In the first embodiment, the transparent material is designed such thatlight is propagated through the transmitting lens 1 in a directionparallel to the optical axis of light incident on the VIPA variabledispersion compensator. What is essential is, however, that light isrefracted and emitted from the transmitting lens in the same direction(but opposite sides of the Z axis) as attained by the reflecting mirrorof the conventional device. The transmitting lens may therefore beconfigured differently insofar as it is designed such that light notperpendicular to the optical axis of the incident light is refracted andthen emitted in the same direction as attained by the reflecting mirror.

FIG. 15 shows an exemplary shape of the transmitting lens. Atransmitting lens 80 shown in FIG. 15 has aspherical incidence andemission surfaces 81 and 82.

The incidence surface 81 is concaved at one end thereof on the negativeside of the X axis, gradually becomes straight and then convex along thepositive direction of the X axis, and is convexed at the other endthereof on the positive side of the X axis.

The emission surface 82 is convexed at one end thereof on the negativeside of the X axis, gradually becomes straight and then concave alongthe positive direction of the X axis, and is concaved at the other endthereof on the positive side of the X axis.

In this manner, the contours of the lens are varied along the X-axisdirection (moving direction of the transmitting lens), whereby thedegree of divergence or convergence can be set differently according tothe light incidence position with respect to the X axis.

Third Embodiment

According to a third embodiment, the transmitting lens is moved by adriving motor.

In the first embodiment, the transmitting lens 1 is merely explained asa movable lens; by moving the transmitting lens 1 with the use of amotor or the like, it is possible to control the group delay time bymeans of an external signal.

FIGS. 16A and 16B illustrate a VIPA variable dispersion compensatoraccording to the third embodiment, wherein FIG. 16A is a diagram showingan entire arrangement, and FIG. 16B is an enlarged view of atransmitting lens driving section. The VIPA variable dispersioncompensator of the third embodiment is almost identical with that shownin FIG. 1; therefore, identical reference numerals are used to denoteidentical elements and description of such elements is omitted.

In the third embodiment, the transmitting lens 1 is coupled to a drivingmotor 91 for moving the lens 1. The driving motor 91 has a shaft 92connected to an X-axis side face of the transmitting lens 1. Inaccordance with a signal input from outside, the driving motor 91 movesthe shaft 92 in the X-axis direction. As the shaft 92 moves, thetransmitting lens 1 moves in the X-axis direction, thus shifting theincidence position of light 93.

The transmitting lens 1 is thus equipped with the driving motor 91, andaccordingly, even in cases where the dispersion of light incident on theVIPA variable dispersion compensator varies with time, the group delaytime can be controlled so as to follow up the varying dispersion.Namely, the refracting angle of the transmitting lens 1 continuouslychanges in the X-axis direction, and therefore, by controlling themovement of the lens 1 in the X-axis direction by means of an externalsignal, it is possible to perform group delay time control in real time.

In the individual embodiments of the invention described above, thedispersion function and the delay function are accomplished by differentoptical systems and thus can be individually adjusted with ease. Namely,the dispersion unit 10 is adjusted so as to obtain a desired dispersionfunction, and the group delay generation unit 20 is adjusted so as toobtain a desired group delay function. Consequently, it is possible toproduce with ease a VIPA variable dispersion compensator capable ofperforming a dispersion compensation function as designed.

Also, the dispersion function can be adjusted independently, so that thefree settable range of the dispersion function widens. For example,higher orders of interference fringes may be generated to obtain veryhigh dispersive power (ratio of change in diffraction angle to change inwavelength).

Similarly, the delay function can be adjusted independently, so that thefree settable range of the group delay time widens. For example, thegroup delay time can be increased by increasing the thickness of thedelay section 22.

The transmitting lens 1 is configured such that the angle of refractingthe incident light varies continuously (or stepwise) in a direction(X-axis direction) perpendicular to the dispersing direction, andaccordingly, the group delay time characteristic can be changed bymoving the transmitting lens 1 in the X-axis direction. For example, theinclination of the linear function representing the group delay timecharacteristic may be varied continuously (or stepwise).

Further, the transmitting lens 1 can be moved by the driving motor, andin this case, the group delay time characteristic can be controlled bymeans of external signal.

Moreover, the dispersion unit 10, the transmitting lens 1 and the groupdelay generation unit 20 are arranged along the optical axis of lightincident on the VIPA variable dispersion compensator, whereby lightparallel with the optical axis of the incident light enters thetransmitting lens 1. By thus arranging the individual elements along astraight line, it is possible to reduce the overall size of the device.

The foregoing embodiments are illustrative only of the presentinvention. The present invention can therefore be modified in variousways by those skilled in the art on the basis of the principles of thepresent invention and without departing from the scope of the inventionin the appended claims and their equivalents.

As described above, according to the present invention, the incidentlight dispersion function is separated from the group delay generationfunction for subjecting dispersed light beams of respective wavelengthsto propagation delay, and the intervening transmitting lens is used toadjust the group delay time to be generated. Accordingly, the amount ofdispersion and the group delay time can be adjusted separately, thusmaking it easy to obtain desired characteristics.

The foregoing is considered as illustrative only of the principles ofthe present invention. Further, since numerous modifications and changeswill readily occur to those skilled in the art, it is not desired tolimit the invention to the exact construction and applications shown anddescribed, and accordingly, all suitable modifications and equivalentsmay be regarded as falling within the scope of the invention in theappended claims and their equivalents.

1. A chromatic dispersion compensation device to correct wavelengthcorrelated dispersion of light transmitted through an optical fiber,comprising: a dispersion unit for separating light emerging from theoptical fiber in a propagation direction into light beams of differentwavelengths dispersed in a first direction; a transmitting lens whichrefracts the light beams received at different incidence positions inthe first direction, at different angles with respect to a seconddirection perpendicular on the first direction and on the propagationdirection, wherein the transmitting lens position is movable foradjustment in the first direction; and a group delay generation unitreceiving the light beams from said transmitting lens, for addingpropagation delays corresponding to incidence positions of the lightbeams on said group delay generation unit, and outputting the lightbeams of the different wavelengths.